The present invention relates to ink jet printing, and in particular to an improved method for positioning dots produced by a continuous ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging plate, which charges individual drops at a selected voltage. The drops then pass through a transverse electric field provided across a pair of deflection plates. Each drop is deflected by an amount which depends on its charge. If the drop is uncharged, it will pass through the deflection plates without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate is moving past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
It is general practice to provide predefined raster patterns, with the matrix for each pattern, customarily representing a character, of a predetermined size. For example, a 5 high by 5 wide matrix representing an image, as shown in FIG. 2A, can be created which represents a whole image such as a character or a portion of an image. A technique which has become widely used for printing these characters or portions of images is disclosed in U.S. Pat. No. 3,298,030 (Lewis et al). A stroke is defined for each column of the matrix and represents a slice of the image. Each usable drop is assigned to each pixel (dot position) in the stroke. If the pixel is a blank pixel, then the drop is not charged and is captured by the catcher to be sent back to the ink supply. If the pixel is to be printed, an appropriate charge is put on the drop so that it is deflected to follow a trajectory that intercepts the substrate at the appropriate position in the column for that stroke. This cycle repeats for all strokes in a character and then starts again for the next character. If the drops are deflected transversely to the direction of travel of the substrate, a set of drops forming a stroke will clearly lie along a diagonal line, as the substrate will move a certain distance between each drop in the stroke. The angular deviation of the line from vertical will increase with the speed of the substrate relative to the drop emission rate. This angular deviation can be counteracted by angling the deflection plates away from the vertical direction by an amount dependent on the expected speed of the substrate. If drops in a stroke are not sequentially allocated to equally spaced positions on the substrate, the points will no longer lie along a straight line. In order to maintain a simple matrix raster pattern, with straight lines in any direction in the matrix mapping onto straight lines on the substrate, it is necessary to print drops in a stroke sequentially with an equal time interval between each stroke. A stroke takes the same time whether it contains one printed drop or five printed drops. Generally, a varying number of extra guard drops are used at the end of each stroke to permit variation in the substrate speed on a stroke by stroke basis.
It is well known that character definition improves with more dot positions in a vertical stroke and more strokes per character. FIGS. 2A and 2B respectively show characters based on 5×5 and 7×9 matrices. The 7×9 matrix clearly yields better defined characters. However, for a constant drop rate determined by the limitations of the hardware, in order to be able to print at all pixel locations in the matrix, the maximum substrate speed will have to be inversely proportional to the number of pixels per character. Traditionally, improved character definition required reducing the maximum substrate speed. Conversely, a smaller matrix allows increased line speed, but the characters become less defined. There is a conflicting need for better defined fonts at higher speeds, which are still formed from a simple orthogonal matrix.
An approach which has been used to improve character definition while maintaining the same stroke rate is described in U.S. Pat. No. 4,115,787 (Fujimoto et al.). While dots are generally printed along a conventional stroke, each dot can optionally be vertically deflected to a different location approximately half a stroke height away. At this time, the print head will have moved approximately half a stroke width in the direction of travel of the substrate relative to a dot on the previous stroke at the same vertical position. This therefore gives a way of printing dots along a “virtual” stroke horizontally between two successive conventional strokes. A significant disadvantage of this technique is that varying the number of guard drops between strokes will have a significant effect on the continuity of the dots in an interpolated stroke. A further disadvantage of this technique is that it is very difficult to establish allowable dot patterns in a font, as there are two allowable dot positions for any particular drop which are substantially separated on the grid of allowable dot positions.
U.S. Pat. No. 6,109,739 (Stamer et al.), owned by the assignee of the present application, discloses another approach for improving character definition while maintaining line speed. The '739 patent provides a print method in which a set number of virtual drop positions (N) are assigned to a stroke, but in which the number of drops that can be printed (n) is less than the number of positions on the stroke. One example disclosed in the '739 patent is a 5×9 font, wherein each stroke has 9 virtual positions, but no more than 5 drops can be printed in a stroke. As can be seen in FIG. 3, the print method of the '739 patent provides improved resolution at the same print speed (e.g., compare FIG. 3 to FIG. 2A).
However, applying the print method of the '739 patent to multiple line text, such as a twin line print application, or to large fonts, such as 16 high or 24 high fonts, has practical limitations. As is discussed in greater detail in the '739 patent (the disclosure of which is hereby incorporated by reference), each drop in the ink jet stream interacts with the other drops in a complex fashion. In particular, any two charged drops have an electrostatic force given by the well-known relation:                     F        ∝                              q1            ×            q2                                r            2                                              (        1        )            
where q1 and q2 are the electrostatic charges on the two drops, r is the distance between the centers of the two drops and F is the electrostatic force between drops referred to as “drop interaction.” This interaction is further complicated by the aerodynamic effects caused by air disturbance due to drops preceding the print drop. Hence, as discussed in the '739 patent, the voltage applied to a print drop is typically compensated for electrostatic and aerodynamic effects based on its interaction with the other drops in the respective stroke. These compensations, which must generally be empirically determined, are time consuming and labor intensive to perform. In a single line application of a small to medium font these compensations are practical to perform and execute during operation. For example, a stroke according to the '739 patent which has 9 virtual print positions only results in 29 (or 512) possible drop combinations. However, it is not feasible or practical to compensate, test, and store all of the possible drop combinations that result when the '739 method is applied to multiple line applications or to large fonts. For example, a twin line application with 9 virtual positions per line results in 218 (or 262,144) possible drop combinations for which the voltage compensations are needed. These 218 possible combinations may in turn require over 2.6 million bytes of processor memory, e.g. 264,144 possible strokes of 10 drops each. This greatly exceeds the memory capacity of the processors typically employed in continuous ink jet printers, particularly where cost is a limiting factor in the design of the printer.